Electromotive Forces in the Voltaic Cell. 341 
contact between metal and medium, rather than between metal 
and metal, it remains to consider whether this belief requires 
one to assert that there is no true contact-force at all at the 
junction of two metals. By no means: the existence of such 
a force is undoubted ; but for metals it is usually very small 
and may be neglected in comparison with the Volta-force, 
though, strictly speaking, what is observed electroscopically 
is a mixture of the two. It is the true contact-force which 
gives rise to the Peltier effect, and its variation with tempe- 
rature (assisted by the Thomson effect) causes thermoelectric 
currents. A contact-force exists, as Thomson has shown, not 
only at the junction of two different metals, but also between 
parts of the same metal at different temperatures. 
In another place* I have endeavoured to gain some insight 
into the nature of this true contact-force and to suggest its 
cause. This has been done by many others; but I may be 
permitted to repeat my own notion, vague and incomplete 
though it avowedly is. Molecules of matter do not move in 
independence of electricity ; at any rate the converse is cer- 
tainly true—electricity does not move independently of matter. 
Hlectricity, in flowing through a wire, meets with resistance ; 
there is something analogous to friction between the matter 
and the electricity, and the opposing force is precisely pro- 
portional to the strength of the current. This much is 
expressed by Ohm’s law, H= RQ, which is a carefully verified 
though empirical statement. But, analyzing R into specific 
resistance of material (p) and sectional area of conductor, and 
permitting ourselves to regard ee as proportional to the 
velocity of electricity in a circuit of different thicknesses, we 
perceive that Ohm’s law means that 
ANS): eae 
dg =P% Velocity. 
Let us then postulate, between electricity and any given 
kind of conducting matter, a connection which shows itself as 
an E.M.F. proportional to the speed of their relative motion 
and to the specific resistance of the material. Molecules of 
matter are not at rest, but (say ) vibrating at a rate depending on, 
or rather itself determining, the temperature. These motions, 
as we have seen, cannot be independent of electricity, but 
they result in no force urging it to flow, because their motions 
are symmetrical. But place two metals in contact—one hot, 
the other cold ; or one copper, the other iron: at the junction 
Pea Aehil: Mag. December (Supplement) 1876, “On a Mechanical Dlus- 
tration of Thermoelectric Phenomena.” 
